Process of electroplating chromium



I Patented Mar. 27, 1934 UNITED STATES PATENT OFFICE No Drawing.

Original application January 3,

1927, Serial No. 158,822. Patent No. 1,864,014, dated June 21, 1932. Divided and this application March 12, 1928, Serial No. 261,181

7 Claims.

The invention relates to electrolytes for electrodeposition of chromium and to a process for electroplating chromium in a form suitable for commercial requirements.

This application is a divisional of my co-pending application on Chromium plating process, Serial No. 158,822 filed January 3, 1927 Patent No. 1,864,014, June 21, 1932.

In my co-pending application, Serial No. 126,094 filed July 30, 1926 Patent No. 1,864,013, June 21, 1932 I have described a method for rendering a solution of chromic acid suitable for electrodeposition of chromium by the addition of certain compounds of metals having oxidizing and reducing properties and at least two valent states. I have now discovered that it is preferable to use such metals as are found in the second and third groups of the periodic system and which are electronegative to chromium. Such metals are alu- 20 minum, zinc, magnesium, cerium, lanthanum, praseodymium and the remaining elements of the rare earth group. The element of the rare earth group which is particularly advantageous is cerium.

In the preferred embodiment of the invention the electrolyte consists of a soluble chromium compound such as chromic acid, which in itself is not suitable for electrodeposition of chromium and the electrolyte is rendered suitable for depositing chromium by the addition of a cerium compound, such for example as cerium oxalate, cerium carbonate, etc. Although chromium may be plated from such solutions the results at times are irregular, due to the formation of a brownish film in spots upon the surface of the chromium. Where such areas are found, the deposits of chromium seem to be thinner and under certain conditions the film upon drying the plated object is quite adherent and bufling is required for its removal. I have discovered that this film may be prevented by adding to the electrolyte certain additional substances, preferably salts of a metal having a low hydrogen overvoltage.

The quantity of this material which should be used can not be well stated in terms of weights and volumes because it is a function of such factors as current density, temperature of bath, concentration of chromic acid, proportion of cerium oxalate or other compound added to the chromic acid, the anode material and the shape of the object to be plated. Therefore the amount of the film-preventing substance which should be added to the electrolyte can best be found by trial, although it may be stated briefly that the 55 amount used should be that quantity which is just necessary to prevent the formation of the film when the operator is working with a clean piece of work, such as an object of brass, steel or nickel. If substantially more than this amount is added, the efiiciency of the plating process is 0 somewhat lowered. As an example of some of the substances which have been found to prevent the formation of the brownish film are the following: Hydrofiuosilicic acid, hydrofluoboric acid, or their soluble salts, sulphites, or persulphates.

The material which I have found best suited for preventing the brownish film is a soluble persulphate such as the potassium sodium or ammonium salt. This substance has no chemical effect upon the anodes used in the process and the materials used for the anodes can therefore be chosen to best suit other conditions. Steel or lead anodes are recommended for the process, although other anodes are also suitable.

Instead of adding the persulphate in the form indicated above, it is also possible to introduce the same as a salt of the metals of the second and third groups heretofore enumerated. In such cases it is necessary to add to the chromic acid solution only one compound, for example 0 cerium persulphate in order to obtain an electrolyte suitable for electrodeposition of chromium without formation of any brownish film.

It is preferable, however, to add the cerium and the persulphate as separate compounds so that the amounts of the respective substances added need not be dependent upon each other. As mentioned above, the amount of persulphate or equivalent substance added varies with different conditions and it is preferable to have only such amount added as to overcome the formation of the brownish film.

As an example of a specific electrolyte suitable for commercial requirements and the process of preparing the solution, the following formula is given:

Add to a liter of water 200 to 300 grams of commercial chromic acid. When this has dissolved, 10 grams of commercial cerium oxalate is added, with stirring, and dissolved. This solution is now treated with a dilute solution of sodium persulphate about 1 gram per 100 cc. solution until upon making a trial under the best current density by plating a brass or nickel object (area about 1 square decimeter) no brown film is formed. The temperature of the; bath may be about 20 C.

While in the above formula I have suggested the use of cerium oxalate as a substance for rendering the chromic acid bath suitable for electroplating, it is to be understood that my invention can also be carried out by adding the film-preventing substance to a chromic acid bath having incorporated therein other compounds of cerium such as the oxides, carbonates, sulphates, etc. and in the broader aspects of the invention the film-preventing compound may be added to any solution of a chromium compound which has incorporated therein a compound of a metal having oxidizing and reducing properties and at least two valent states. The addition of the sodium persulphate or the equivalent substances above enumerated to the electrolyte results in an improved deposit of chromium on the object, this deposit being of uniform thickness, bright and lustrous and free from any brownish film. It is believed that one of the reasons why these inorganic substances have such a beneficial action on the electrolyte is partly due to the fact that they are colloidal. It is believed essential that the substance be inorganic in character rather than organic because the presence of chromic acid in the electrolyte introduces such strong oxidizing properties that organic matter present is destroyed.

In carrying out the process of electrodeposition of chromium utilizing the improved electrolyte above described, it has been found very advantageous to accurately determine the amount of current in the working circuit, especially when the bath is being used for objects of different shapes or materials. It is therefore desirable to have an ammeter arranged in the working circuit and by using the ammeter I have discovered a method by which the emciency of the electroplating process may be increased and the proper current density determined for any particular object which is to be plated. ,My method consists in placing the object in the electrolyte with a small current flowing in the working circuit and gradually increasing the effective voltage on the bath by rheostats or other suitable means and carefully noting the resulting increase of current flowing through the circuit. It has been found that as the voltage is gradually increased the current also is gradually and uniformly increased until a certain critical point is reached where there is a sudden and appreciable drop in the amperage as indicated by the ammeter. It has been found that when the object is plated with the current corresponding to the critical point or a current just slightly greater than the critical point, a verysatisfactory deposit of chromium is obtained and the efficiency of the bath is higher than with other current densities.

It is to be understood that the electroplating process as above described can be carried out on various surfaces such as copper, cadmium, nickel, zinc, etc. and various alloys of these metals as well as iron and steel. The surfaces may be either polished or unpolished, but if it is desired to obtain a bright polished chromium surface it is preferable to have a polished under surface on which the deposition takes place to avoid subsequent smoothing of, the extremeli hard chromium plate.

What I claim as my invention is:

1. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid as the principal ingredient, a relatively small amount of a salt of a heavy metal of the second or third group which is electronegative to chromium and a relatively small amount of a soluble persulphate.

2. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid as the principal ingredient, a relatively small amount of a cerium compound and a relatively small amount of a soluble persulphate.

3. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid substantially free from trivalent chromium and a relatively small amount of a compound of a heavy metal of the second and third groups electronegative to chromium capable of maintaining said electrolyte substantially free from trivalent chromium during electrolysis.

4. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid substantially free from trivalent chromium, a relatively small amount of a compound of a metal of the second and third groups electronegative to chromium capable of maintaining said electrolyte substantially free from trivalent chromium during electrolysis and a relatively small amount of a soluble persulphate.

5. An electrolyte for electrodeposition of chro mium comprising a solution containing chromic acid asthe principal ingredient and an amount of a soluble persulphate sufficient to prevent the formation of a brownish film.

6. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid substantially free from trivalent chromium and a relatively small amount of a soluble persulphate. I

'7. An electrolyte for electrodeposition of chromium comprising a solution containing chromic acid as the principal ingredient, a relatively small amount of a cerium compound and an amount of a soluble persulphate sufiicient to prevent the formation of a brownish film.

DWIGHT T. EWING. 

